Ordering Choropleth Map Symbols: The Effect of Background

Guidelines for choropleth map design have concentrated on printed maps. This paper examines whether the conventional use of darker symbols to mean "more" is applicable on maps presented in other media. Forty-seven subjects performed map reading tasks using slides photographed from a CRT screen. Results indicate that map readers take darker symbols for "more" in these displays. Further, the tendency for map readers to take darker symbols to represent greater magnitudes is modified by the value of a map's background. Approximately one-fifth of the experimental subjects reversed symbol order because of background value. Backgrounds of intermediate value impeded symbol ordering. This evidence suggests that for at least some subjects, a map's background anchors map symbols and influences their apparent order.